In recent years, multi-window systems are popular in personal computers and work stations.
Specific display regions on the display screen of a single image display apparatus are respectively assigned to a plurality of application programs on the multi-window system. Furthermore, an OS (operating system) of the multi-window system executes processing for opening/closing a file, and processing for opening or closing image processing associated with the plurality of display regions, i.e., "windows" so as to start/end a process according to a request from an application program executed by a personal computer.
In a general multi-window system, a window interface with a user operating a system (i.e., a window selected by the user) is processed as an active window, and is displayed at the top position of a plurality of windows. Therefore, the entire display region of the active window is displayed. The display regions of some windows other than the active window are often locally concealed under other windows. The OS of the multi-window system performs overlay-display control, i.e., hierarchical display control of the windows in addition to the basic open/close control of the windows.
In most personal computers and work stations, which load multi-window systems, pixel data is stored in a single bit-map output image memory, and is displayed on the screen of an image display apparatus. For this reason, exclusive display control of portions concealed under other windows is normally only executed by the OS of the multi-window system.
In recent years, as representative multi-media information, personal computers, which process dynamic images, and systems called multi-media terminals have become available. Various methods of overlay-displaying a dynamic image on a given window in these image processing apparatuses which load the multi-window systems have been proposed. In most of these methods, a special pixel data pattern for overlay-display of a dynamic image is predetermined, and a portion inside the window to be subjected to the overlay-display is painted by the pixel data pattern.
Furthermore, in a multi-media terminal, which loads an application of a television (TV) telephone, when two dynamic image sources, i.e., its own dynamic image (own image) and a dynamic image (received image) sent from a terminal on the other end of a line are to be processed, the multi-window system must display a plurality of dynamic images on windows.
As described above, when overlay-display control of windows is performed by only an OS, or when a dynamic image is overlay-displayed on output images by painting a portion inside a given window by a specific pixel data pattern, when high-speed image development to a memory, in particular, the overlay-display of a plurality of dynamic images on windows must be performed. If read/write accesses of image data to the memory and transfer processing of image data to an image display apparatus are executed by the OS, i.e., program commands, the OS is overloaded, and a satisfactory display speed cannot be obtained.